Reduced with m-c:
BuildID=20190702093917
SourceStamp=a15e5a44b7ed0a285c92839055efd83671b49552

[Child 106278, Main Thread] ###!!! ASSERTION: Shouldn't be incomplete if availableBSize is UNCONSTRAINED.: 'aReflowInput.AvailableBSize() != NS_UNCONSTRAINEDSIZE', file src/layout/generic/nsBlockFrame.cpp, line 1879
Assertion failure: aListID != kPrincipalList || GetInner() (Setting principal child list should populate our inner frame), at src/layout/forms/nsFieldSetFrame.cpp:666

#0 nsFieldSetFrame::SetInitialChildList(mozilla::layout::FrameChildListID, nsFrameList&) src/layout/forms/nsFieldSetFrame.cpp:665:3
#1 SetInitialSingleChild(nsContainerFrame*, nsIFrame*) src/layout/base/nsCSSFrameConstructor.cpp:610:12
#2 nsCSSFrameConstructor::CreateContinuingFrame(nsPresContext*, nsIFrame*, nsContainerFrame*, bool) src/layout/base/nsCSSFrameConstructor.cpp
#3 nsBlockFrame::CreateContinuationFor(mozilla::BlockReflowInput&, nsLineBox*, nsIFrame*) src/layout/generic/nsBlockFrame.cpp:4486:22
#4 nsBlockFrame::ReflowBlockFrame(mozilla::BlockReflowInput&, nsLineList_iterator, bool*) src/layout/generic/nsBlockFrame.cpp:3853:35
#5 nsBlockFrame::ReflowLine(mozilla::BlockReflowInput&, nsLineList_iterator, bool*) src/layout/generic/nsBlockFrame.cpp:3050:5
#6 nsBlockFrame::ReflowDirtyLines(mozilla::BlockReflowInput&) src/layout/generic/nsBlockFrame.cpp:2592:7
#7 nsBlockFrame::Reflow(nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, nsReflowStatus&) src/layout/generic/nsBlockFrame.cpp:1341:3
#8 nsContainerFrame::ReflowChild(nsIFrame*, nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, mozilla::WritingMode const&, mozilla::LogicalPoint const&, nsSize const&, unsigned int, nsReflowStatus&, nsOverflowContinuationTracker*) src/layout/generic/nsContainerFrame.cpp:894:14
#9 nsFieldSetFrame::Reflow(nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, nsReflowStatus&) src/layout/forms/nsFieldSetFrame.cpp:536:5
#10 nsBlockReflowContext::ReflowBlock(mozilla::LogicalRect const&, bool, nsCollapsingMargin&, int, bool, nsLineBox*, mozilla::ReflowInput&, nsReflowStatus&, mozilla::BlockReflowInput&) src/layout/generic/nsBlockReflowContext.cpp:297:11
#11 nsBlockFrame::ReflowBlockFrame(mozilla::BlockReflowInput&, nsLineList_iterator, bool*) src/layout/generic/nsBlockFrame.cpp:3687:11
#12 nsBlockFrame::ReflowLine(mozilla::BlockReflowInput&, nsLineList_iterator, bool*) src/layout/generic/nsBlockFrame.cpp:3050:5
#13 nsBlockFrame::ReflowDirtyLines(mozilla::BlockReflowInput&) src/layout/generic/nsBlockFrame.cpp:2592:7
#14 nsBlockFrame::Reflow(nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, nsReflowStatus&) src/layout/generic/nsBlockFrame.cpp:1341:3
#15 nsBlockReflowContext::ReflowBlock(mozilla::LogicalRect const&, bool, nsCollapsingMargin&, int, bool, nsLineBox*, mozilla::ReflowInput&, nsReflowStatus&, mozilla::BlockReflowInput&) src/layout/generic/nsBlockReflowContext.cpp:297:11
#16 nsBlockFrame::ReflowBlockFrame(mozilla::BlockReflowInput&, nsLineList_iterator, bool*) src/layout/generic/nsBlockFrame.cpp:3687:11
#17 nsBlockFrame::ReflowLine(mozilla::BlockReflowInput&, nsLineList_iterator, bool*) src/layout/generic/nsBlockFrame.cpp:3050:5
#18 nsBlockFrame::ReflowDirtyLines(mozilla::BlockReflowInput&) src/layout/generic/nsBlockFrame.cpp:2592:7
#19 nsBlockFrame::Reflow(nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, nsReflowStatus&) src/layout/generic/nsBlockFrame.cpp:1341:3
#20 nsContainerFrame::ReflowChild(nsIFrame*, nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, mozilla::WritingMode const&, mozilla::LogicalPoint const&, nsSize const&, unsigned int, nsReflowStatus&, nsOverflowContinuationTracker*) src/layout/generic/nsContainerFrame.cpp:894:14
#21 nsCanvasFrame::Reflow(nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, nsReflowStatus&) src/layout/generic/nsCanvasFrame.cpp:730:5
#22 nsContainerFrame::ReflowChild(nsIFrame*, nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, mozilla::WritingMode const&, mozilla::LogicalPoint const&, nsSize const&, unsigned int, nsReflowStatus&, nsOverflowContinuationTracker*) src/layout/generic/nsContainerFrame.cpp:894:14
#23 nsHTMLScrollFrame::ReflowScrolledFrame(mozilla::ScrollReflowInput*, bool, bool, mozilla::ReflowOutput*) src/layout/generic/nsGfxScrollFrame.cpp:625:3
#24 nsHTMLScrollFrame::ReflowContents(mozilla::ScrollReflowInput*, mozilla::ReflowOutput const&) src/layout/generic/nsGfxScrollFrame.cpp:773:7
#25 nsHTMLScrollFrame::Reflow(nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, nsReflowStatus&) src/layout/generic/nsGfxScrollFrame.cpp:1140:3
#26 nsContainerFrame::ReflowChild(nsIFrame*, nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, int, int, unsigned int, nsReflowStatus&, nsOverflowContinuationTracker*) src/layout/generic/nsContainerFrame.cpp:933:14
#27 mozilla::ViewportFrame::Reflow(nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, nsReflowStatus&) src/layout/generic/ViewportFrame.cpp:307:7
#28 mozilla::PresShell::DoReflow(nsIFrame*, bool, mozilla::OverflowChangedTracker*) src/layout/base/PresShell.cpp:9296:11
#29 mozilla::PresShell::ProcessReflowCommands(bool) src/layout/base/PresShell.cpp:9466:24
#30 mozilla::PresShell::DoFlushPendingNotifications(mozilla::ChangesToFlush) src/layout/base/PresShell.cpp:4232:11
#31 mozilla::PresShell::DoFlushPendingNotifications(mozilla::FlushType) src/layout/base/PresShell.cpp:4010:3
#32 nsDocumentViewer::LoadComplete(nsresult) src/layout/base/nsDocumentViewer.cpp:1016:16
#33 nsDocShell::EndPageLoad(nsIWebProgress*, nsIChannel*, nsresult) src/docshell/base/nsDocShell.cpp:6677:20
#34 nsDocShell::OnStateChange(nsIWebProgress*, nsIRequest*, unsigned int, nsresult) src/docshell/base/nsDocShell.cpp:6477:7
#35 non-virtual thunk to nsDocShell::OnStateChange(nsIWebProgress*, nsIRequest*, unsigned int, nsresult) src/docshell/base/nsDocShell.cpp
#36 nsDocLoader::DoFireOnStateChange(nsIWebProgress*, nsIRequest*, int&, nsresult) src/uriloader/base/nsDocLoader.cpp:1337:3
#37 nsDocLoader::doStopDocumentLoad(nsIRequest*, nsresult) src/uriloader/base/nsDocLoader.cpp:896:14
#38 nsDocLoader::DocLoaderIsEmpty(bool) src/uriloader/base/nsDocLoader.cpp:730:9
#39 nsDocLoader::OnStopRequest(nsIRequest*, nsresult) src/uriloader/base/nsDocLoader.cpp:618:5
#40 non-virtual thunk to nsDocLoader::OnStopRequest(nsIRequest*, nsresult) src/uriloader/base/nsDocLoader.cpp
#41 mozilla::net::nsLoadGroup::RemoveRequest(nsIRequest*, nsISupports*, nsresult) src/netwerk/base/nsLoadGroup.cpp:568:22
#42 imgRequestProxy::RemoveFromLoadGroup() src/image/imgRequestProxy.cpp:404:15
#43 imgRequestProxy::OnLoadComplete(bool) src/image/imgRequestProxy.cpp:1048:7
#44 void mozilla::image::ImageObserverNotifier<mozilla::image::ObserverTable const*>::operator()<void mozilla::image::SyncNotifyInternal<mozilla::image::ObserverTable const*>(mozilla::image::ObserverTable const* const&, bool, unsigned int, mozilla::gfx::IntRectTyped<mozilla::gfx::UnknownUnits> const&)::'lambda5'(mozilla::image::IProgressObserver*)>(mozilla::image::ObserverTable const*) src/image/ProgressTracker.cpp:283:9
#45 void mozilla::image::SyncNotifyInternal<mozilla::image::ObserverTable const*>(mozilla::image::ObserverTable const* const&, bool, unsigned int, mozilla::gfx::IntRectTyped<mozilla::gfx::UnknownUnits> const&) src/image/ProgressTracker.cpp:352:5
#46 mozilla::image::ProgressTracker::SyncNotifyProgress(unsigned int, mozilla::gfx::IntRectTyped<mozilla::gfx::UnknownUnits> const&)::$_1::operator()(mozilla::image::ObserverTable const*) const src/image/ProgressTracker.cpp:371:5
#47 decltype(fp(static_cast<mozilla::image::ObserverTable const*>(std::nullptr_t))) mozilla::image::CopyOnWrite<mozilla::image::ObserverTable>::Read<mozilla::image::ProgressTracker::SyncNotifyProgress(unsigned int, mozilla::gfx::IntRectTyped<mozilla::gfx::UnknownUnits> const&)::$_1>(mozilla::image::ProgressTracker::SyncNotifyProgress(unsigned int, mozilla::gfx::IntRectTyped<mozilla::gfx::UnknownUnits> const&)::$_1) const src/image/CopyOnWrite.h:155:12
#48 mozilla::image::ProgressTracker::SyncNotifyProgress(unsigned int, mozilla::gfx::IntRectTyped<mozilla::gfx::UnknownUnits> const&) src/image/ProgressTracker.cpp:370:14
#49 mozilla::image::RasterImage::NotifyProgress(unsigned int, mozilla::gfx::IntRectTyped<mozilla::gfx::UnknownUnits> const&, mozilla::Maybe<unsigned int> const&, mozilla::image::DecoderFlags, mozilla::image::SurfaceFlags) src/image/RasterImage.cpp:1620:28
#50 mozilla::image::RasterImage::NotifyForLoadEvent(unsigned int) src/image/RasterImage.cpp:938:3
#51 mozilla::image::RasterImage::OnImageDataComplete(nsIRequest*, nsISupports*, nsresult, bool) src/image/RasterImage.cpp:921:3
#52 imgRequest::OnStopRequest(nsIRequest*, nsresult) src/image/imgRequest.cpp:787:16
#53 nsJARChannel::OnStopRequest(nsIRequest*, nsresult) src/modules/libjar/nsJARChannel.cpp:1026:16
#54 non-virtual thunk to nsJARChannel::OnStopRequest(nsIRequest*, nsresult) src/modules/libjar/nsJARChannel.cpp
#55 nsInputStreamPump::OnStateStop() src/netwerk/base/nsInputStreamPump.cpp:655:16
#56 nsInputStreamPump::OnInputStreamReady(nsIAsyncInputStream*) src/netwerk/base/nsInputStreamPump.cpp:403:21
#57 non-virtual thunk to nsInputStreamPump::OnInputStreamReady(nsIAsyncInputStream*) src/netwerk/base/nsInputStreamPump.cpp
#58 nsInputStreamReadyEvent::Run() src/xpcom/io/nsStreamUtils.cpp:91:20
#59 nsThread::ProcessNextEvent(bool, bool*) src/xpcom/threads/nsThread.cpp:1225:14
#60 NS_ProcessNextEvent(nsIThread*, bool) src/xpcom/threads/nsThreadUtils.cpp:486:10
#61 mozilla::ipc::MessagePump::Run(base::MessagePump::Delegate*) src/ipc/glue/MessagePump.cpp:88:21
#62 MessageLoop::RunInternal() src/ipc/chromium/src/base/message_loop.cc:315:10
#63 MessageLoop::Run() src/ipc/chromium/src/base/message_loop.cc:290:3
#64 nsBaseAppShell::Run() src/widget/nsBaseAppShell.cpp:137:27
#65 XRE_RunAppShell() src/toolkit/xre/nsEmbedFunctions.cpp:919:20
#66 mozilla::ipc::MessagePumpForChildProcess::Run(base::MessagePump::Delegate*) src/ipc/glue/MessagePump.cpp:238:9
#67 MessageLoop::RunInternal() src/ipc/chromium/src/base/message_loop.cc:315:10
#68 MessageLoop::Run() src/ipc/chromium/src/base/message_loop.cc:290:3
#69 XRE_InitChildProcess(int, char**, XREChildData const*) src/toolkit/xre/nsEmbedFunctions.cpp:754:34
#70 content_process_main(mozilla::Bootstrap*, int, char**) src/browser/app/../../ipc/contentproc/plugin-container.cpp:56:28
#71 main src/browser/app/nsBrowserApp.cpp:267:18

The MOZ_ASSERT call stack is about creating a continuation for a multicol <fieldset> if column-spanis enabled. We might not create the multicol correctly if ColumnSetWrapper is the -moz-fieldset-content.

What's interesting is that if column-span is disabled, we don't create any continuation for <fieldset>. Note that there's no column-span children in the test case, and the soft assertion prior to the call stack

[Child 106278, Main Thread] ###!!! ASSERTION: Shouldn't be incomplete if availableBSize is UNCONSTRAINED.: 'aReflowInput.AvailableBSize() != NS_UNCONSTRAINEDSIZE', file src/layout/generic/nsBlockFrame.cpp, line 1879

is a hint that introducing ColumnSetWrapperFrame could change the reflow logic, and is likely the root cause that we need to create such a continuation. We need to invesigate whether this is necessary.

This is fixed by bug 1575106, and the testcase is added as a crashtest in bug 1575106 Part 4.